ACV is an acid in its purest form, but when digested creates a highly alkalitic ash that can be used to neutralize or weaken harmful acidic ashes created by digestion of things like dairy, meats, cocoa, sugar, etc. It's also very beneficial for bone and tissue health, for it contains MANY nutrients and bountiful amounts of minerals that are considered by modern medicine to be "trace minerals" in most foods.
Unless you tear your leg open and pour ACV on the bone, I very highly doubt you'll be doing any damage to your bone structure. The acid is neutralized by the body's bicarbonate stores and then digested. If it's leaking into your knees, you have more serious problems than just pH distortion. : )
BTW: ACV contains such things as calcium NATURALLY. It has calcium in it far before it even makes contact with the human body. In very EXTREME cases of Acidosis of the body (unisolated cases; not just the blood stream, urine, etc), acidic ash content may be neutralized by the extra calcium stores of the body. If you don't have these stores and are overtly malnutritive, calcium will be taken from your bones to neutralize certain acids. Do keep in mind, however, that many, many other alkalitic nutrients are used as a neutralizer (acid weakener) before the calcium in your bones.
Dairy: >>Generally<< bad for you.
Apple Cider Vinegar: One of the most beneficial and cheapest REAL medicines a person can buy that isn't a drug.
Funny that you mention Hippocrates, though. He's historical proof that good nutrition and intelligent nutritional supplementation can solve a great deal of problems. In my person opinion, if you're eating correctly, getting enough sleep (at the right times), getting enough sunlight, and have (at least) reasonably light habitual sanitation practices... You will not have problems with 'disease'.
I do wish you all great luck with whatever troubles you, and for those giving support, make sure to research the subject of your post thoroughly in order to minimize probability of error. : )
Love, light, peace;
Kirska